Absorptive hypercalciuria is a major cause of urolithiasis, encountered in approximately 50% of patients with recurrent formation of kidney stones. The objective of this project is to test the hypothesis that UroPhos-K , a slow-release neutral potassium phosphate, safely corrects the physiological and physicochemical abnormalities found in stone-formers with absorptive hypercalciuria. We have completed and published a 3 month placebo-controlled study that demonstrateds that UroPhos-K improves the physicochemical and physiological abnormalities in these patients (Breslau JBMR 1995, Breslau JCEM submitted). Unlike thiazide treatment in which the hypocalciuric effect declines over the long term, we have found in 20 patients completing 2 years that the effect is maintained (Heller Urolithiasis 1996, abs JBMR).